First axial AlInN UV core-shell nanowire LEDS on silicon
NJIT team UV emitters can be directly used in several practical applications, including UV curing, phototherapy, water/air/surface purification and disinfection.
Until recently, InGaN and AlGaN have been intensively studied for visible and UV photonic devices, while different III-nitride UV materials have been relatively unexplored. Now researchers from the New Jersey Institute of Technology (NJIT) have reported the world’s first axial AlInN UV core-shell nanowire LEDs grown on silicon substrate.
The AlInN nanowire LEDs exhibit a high internal quantum efficiency of ~ 52 percent at room temperature for emission at ~295 nm and peak emission wavelength could be varied from ~290 nm to 355 nm by changing the growth parameters. The research findings have just been published in Nature Scientific Reports and Optics Express. According to the team , these UV emitters can be directly used in several practical applications, including UV curing, phototherapy, water/air/surface purification and disinfection.
The difficulties in epitaxial growth of AlInN have resulted from extremely large differences in optimal growth temperatures for InN (~ 450degC) and AlN (~ 800degC) which results in low crystalline quality, inhomogeneity of epilayers and low device performance. The research team led by PhD student Ravi Teja Velpula and Hieu P. Nguyen in the Department of Electrical and Computer Engineering at NJIT has successfully grown dislocation-free and high-quality AlInN nanowires under nitrogen-rich conditions, which provides an alternative approach for the fabrication of new types of high-performance UV light-emitters.
Shown in Figure 1(a), vertically aligned AlInN/GaN core-shell nanowires were grown on Si (111) substrates using radio-frequency plasma-assisted molecular beam epitaxy (PAMBE). Figure 1(b) shows the 45° tilted scanning electron microscopy (SEM) image of a typical AlInN nanowire LED sample showing uniform nanowires on silicon. The growth conditions of the GaN nanowires included a growth temperature of 770degC, a nitrogen flow rate of 1.0 sccm, a forward plasma power of 400 W.
During the epitaxial growth of AlInN segments, the nitrogen flow rate and plasma power were maintained at 2.5 sccm and 400 W, respectively. The In composition in the AlInN active region could be controlled by varying the In and Al beam flux and/or the substrate temperature.
The AlInN nanowire LEDs show excellent current-voltage characteristics with ~5V turn-on voltage. The leakage current was found to be very small which is approximately 1 µA at -8 V. No obvious shift in the peak wavelength was observed in the measured electroluminescence (EL) spectra as shown in Figure 1(c), attributed to the negligible quantum-confined Stark effect in the LED structures, further confirming the high crystalline quality of such AlInN nanowire heterostructures. Moreover, the AlInN nanowires exhibited a relatively high IQE (~52 percent) at room temperature, due to the strong carrier confinement provided by the AlInN shell and nearly intrinsic AlInN core. Further, the light extraction efficiency of these nanowire LEDs are reported to be enhanced to ~56 percent and ~63 percent using square and hexagonal photonic crystal structures respectively.
Future work will be related to the improvement of the light output power by engineering the device structure and arrangement nanowires on the substrate.
'Epitaxial Growth and Characterization of AlInN-Based Core-Shell Nanowire LEDs Operating in the Ultraviolet Spectrum' by Velpula et al' Nature Scientific Reports, 10 (2020) 2547
'Enhancing the Light Extraction Efficiency of AlInN Nanowire Ultraviolet LEDs with Photonic Crystal Structures' by Jain et al; Optics Express 28 (2020) 22909